Pressure and Temperature Dependence of the Static Dielectric Constants of KCl, NaCl, LiF, and MgO

The static dielectric constants and their pressure derivatives of KCl, NaCl, LiF, and MgO have been measured at several temperatures ranging from 15 to 295 °K. The samples were prepared in the form of three-terminal parallel-plate capacitors and the dielectric constants were determined from measurements of the capacitance with a high-precision bridge. In all cases the dielectric constants decrease with temperature as does the magnitude of the pressure dependence. Effects of permanent-dipole impurities in NaCl were seen at temperatures around 20 °K. The data were used to calculate the fixed-volume temperature derivative of the dielectric constants. This quantity ${\left(\frac{\partial \epsilon }{\partial T}\right)}_V$, apparently exhibiting the effects of lattice anharmonicity, has a similar temperature dependence for all four compounds: It increases from its room-temperature value as the temperature is lowered, reaches a maximum value at $\frac{T}{{\Theta }_D}\approx 0.15$ (${\Theta }_D=\mathrm{Debye}\mathrm{temperature}$), and then decreases rapidly toward zero at lower temperatures.